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"Iron-titanium oxide minerals in some igneous rocks of arran"

Abstract

Examination of the opaque minerals in an intrusive sheet of analcite dolerite shows that they consist partly of a cubic solid solution, referred to for brevity as 'magnetite', and partly of a rhombohedral solid solution referred to as 'ilmenite'. Systematic variation of the properties of the 'magnetites' indicates an increase in titanium dontent towards the more slowly cooled central part of the sheet. 'Ilmenite' usually occurs as oriented intergrowths in 'magnetite’ crystals and contains 10-20% ferric oxide in solid solution. Less commonly, ‘ilmenite' also occurs as a discrete phase believed to have crystallised before the formation of the 'magnetite'-'ilmenite' intergrowths, and this seems not to contain ferric oxide in solid solution. 'Magnetite’ -'ilmenite' relationships are considered in terms of an Fe(_3)O9_4)-FeTiO(_3) phase diagram constructed from the experimental data of previous workers. The content of opaque minerals decreases regularly from 9% in the marginal rocks to less than 1% at the centre of the intrusion. The intensity of natural remanent magnetization decreases systematically away from the margins of the sheet and is clearly dependent on the content of 'magnetite' in the intrusion. The intrusion is reversely magnetized and has a mean direction of magnetization that confirms its age as Tertiary in agreement with geological data. However, there is considerable variation in the palaeomagnetic directions within the sheet due to the multiple natures of the intrusive processes. This is especially clear in the inclinations. The contact relations of the intrusion suggest that the reverse magnetization is a primary feature although the possibility of self-reversal cannot be ruled out entirely. Constructional details are given of instruments designed for the accurate automatic measurement of the Curie temperature of a minute ferromagnetic sample, and for the determination of the saturation magnetization of such samples at room temperature.